1. Field of the Invention
This invention relates to an actuator button system for initiating the picture-taking operation of a single-lens reflex (SLR) camera equipped with a through-the-lens (TTL) light metering system. More particularly, it relates to a compound shutter release actuator with an independently manipulable element for preliminarily depressing the shutter release button for a defined part of its travel for improved control of an electronic shutter controlling the exposure in response to the TTL metering system. The invention is particularly applicable to cameras with full aperture TTL metering systems.
2. Description of the Prior Art
In single-lens reflex cameras equipped for TTL metering and electronic shutter control, the metering element is sometimes positioned to receive light reflected from the viewfinder mirror, in which case the light beam travelling toward the metering element is cut off at the moment picture-taking starts, when the viewfinder mirror flips up out of the picture-taking optical path. Even if the viewfinder mirror is a fixed half-mirror, light metering is disturbed when the lens aperture is stopped down from its fully-open position to a preset value, as must be done before the shutter opens. The metering system alone thus cannot control an electronic shutter to provide a proper exposure and it is necessary to memorize the quantity of light from an object to be photographed measured by the metering system just prior to a picture-taking operation. Some known cameras utilize a capacitor for electrical memorization of the light valve, while other employ mechanical memorizing means. In all of them, the memory means is actuated in response to depression of the shutter release button and prior to the actuation of the movable viewfinder mirror and of the lens aperture means. In other words, the rate of depression of shutter button is relied on to provide an adequate time interval between the successive actuations of the memorizing and shutter release operations at different stages of the travel path of the shutter button. Since an interval of several tens to several hundreds of milliseconds is usually required for completion of the memorization operation, however, an unusually rapid depression of the shutter release button may result in an insufficient memorizing time and hence also an improper exposure.